The overhaul and repair of gas turbine engines often requires the refurbishing of internal engine components. These engine elements are normally treated with various coatings including thermal barrier coatings, abradable seals, and hard facing. A commonly refurbished element is the high pressure compressor (HPC) casing. As known, gas turbine engines use an airseal around the engine's high pressure compressor blades to minimize the clearance between the compressor blade lips and the HPC casing. These seals, often referred to as "rubstrips", are abradable so that the blade tips may penetrate the surface without suffering tip damage. The seals comprise either a rubber material bonded to an amalgam of tungsten carbide and bond coat deposited on the casing, or a plasma sprayed abradable coating (typically nickel aluminide) applied over a bond coat to the outer airseal ring encircling the blades, which must be removed during engine refurbishment to allow refinishing of the HPC surface.
The bonded rubber/tungsten carbide seal is removed with scrapers, wire brushes, and wire wheels, and the plasma sprayed coating is removed by grit blasting, grinding or machining. The bond coats of either configuration are then removed chemically. Current requirements are to remove 90 percent of the rubstrip and bond coating during refurbishment.
Similiarly, scheduled aircraft maintenance may include removal of the aircraft's exterior paint coating due to paint deterioration, coating damage, coating buildup from touch-up painting, or the need to access bare aircraft surfaces to facilitate nondestructive inspection. These coatings, however, are highly adhering, durable paints which are formulated to protect the aircraft's external surfaces from corrosion and solid particulate erosion. They withstand extremes in temperatures, damage during unscheduled maintenance, and exposure to ultraviolet radiation. The inherent toughness and durability of these paint coatings make their removal difficult and expensive.
The aircraft industry, both commercial and military, presently removes organic coatings with methylene chloride-based chemical stripping compounds. This is followed by a mechanical abrasion to remove any coating residue. While chemical stripping is effective in removing paint from aircraft surfaces without damage to the surface metal, it does have several disadvantages, including: slow process time, expense, exposing workers to a hazardous environment, premature degradation of the working areas through the use of chemicals, and costly disposal techniques to minimize the environmental impact of the effluent.
A highly effective alternative to chemical and mechanical stripping is the use of high pressure water jets. This technique has been successfully used by USBI, Inc, a subsidiary of United Technologies Corporation, to remove coatings during refurbishing of space hardware under the National Administration and Space Agency's (NASA) Space Shuffle Program. USBI introduced automated, high pressure waterjet coatings removal in the refurbishment of the space shuttle's solid rocket boosters (SRBs) in the early 1980's. The waterjet stripping process is toxin free and allows for an enviornmentally safe proceedure in removing these coating materials. The process is also more effecient in terms of cost, higher removal rates, and less damage to the underlying substrate material.
The key element in these waterjet stripping systems is the waterjet nozzle. The nozzle must be capable of providing a uniform intensity jet to the target surface to ensure even removal of material in the nozzle's swath (path width) and to prevent substrate damage. The present waterjet nozzles, while effective, are not optimum in their performance in that the incident energy delivered to the target surface is not sufficiently uniform.